Patent GB-757516-A (GB)
Title: Improvements in digital differential analyzers
IPC: G06F7/66
Filed: 1953-10-05
Abstract: 757,516. Digital electric calculating-apparatus. BENDIX AVIATION CORPORATION. Oct. 5, 1953 [Dec. 8, 1952], No. 27297/53. Class 106 (1). In a digital calculator, according to Specification 748,267, comprising a set of storage sections, and common computer means arranged to coast therewith to perform integration, each section is adapted to hold at the beginning of a calculation a number representing an initial value for the dependent variable (y), and the computer means is further arranged to coact with each section to derive from this number a new number dependent on the result of the calculation, the new number serving as the initial yvalue at the beginning of a subsequent calculation. The apparatus described is a development of that described in Specification 748,267 and includes a magnetic drum (not shown) having in addition to the Y, R, Z and clock pulse channels (18, 20, 22 and 24 in the present Specification) a fifth channel (16) containing initial yvalues I. This latter channel and the associated loop circuit are similar to the Y channel and yloop described in Specification 748,267, and during normal computing operations, each I- channel integrator memory section co-operates in turn with the loop circuit to produce and record a modified I- value obtained by algebraically adding increments #<SP>I</SP> selected from the dz (called y#x in the present Specification) precession line by #<SP>I</SP> code pulses. When, however, an overflow occurs in the y-loop adder, i.e. this adder produces an output pulse in the highest, P48 position, this pulse causes gate-controlling trigger circuits to be operated to inhibit normal operations during the next computing cycle and, instead, to cause the I- values to be transferred to the Y- channel, so that the next series of computations are performed with new yvalues adjusted in accordance with the previous computations. This enables solutions of differential equations with given boundary conditions to be obtained by an iterative process. Fig. 6 illustrates how the apparatus can be set up to solve the equation d<SP>2</SP>y/dx<SP>2</SP> = ax + e<SP>y</SP> + c where dy/dx = -b at x = O, and y= O at dy/dx = O. Each of the blocks 130, 132, 144 corresponds to a memory section, the upper and lower values therein being the I- and yvalues respectively. The dx and dy inputs and the dz output are represented on the right by lines above, below and at the centre respectively, and #<SP>I</SP> inputs are represented by lines on the left. A solution to the equation is obtained by sections 130, 132, 134 and 138. The output for 132, repred<SP>2</SP>y dy senting - dx or -, forms the dy input to 134 dx<SP>2</SP> dx whose output, representing y, forms the dx input of 138 whose dy input is obtained from its output whereby e<SP>y</SP> is formed. The output for 138, together with that for 130 representing ax, form the dy inputs for 132, the constant c being included in the initial value for x = O, e<SP>yo</SP> + c (where y<SP>o</SP> is an arbitrary number representing the initial value of y). The output of 134, representing y, forms a dy input for a section 140 arranged to provide an output #Yf representing the error in y, or the amount by which it differs from zero when dy/dx = O. The output #Yf is multiplied by a factor m by means of section 142 whose output #Y forms the dx input for 144 (arranged similarly to 138). The output for 144, #<SP>ey</SP>, provides the #I inputs for 132 and 138. The initial values e<SP>yo</SP> are adjusted every time an overflow is obtained for section 136 (indicating that dy/dx has reached zero), the I- value for this section being b less than the capacity E for the Y- channel. Reference is made also to the solution of problems involving a double integral. Specification 751,101 also is referred to.